The instant invention relates to a protection structure for components of an integrated circuit connected to a pad. More particularly, the instant invention provides for such a structure exhibiting a negative resistance, that is, which is triggered at a first overvoltage value and which afterwards permits the overcurrent to flow under a second voltage having a lower value.
FIGS. 1A, 1B and 1C respectively show a section view, a top view and an equivalent circuit diagram of a conventional integrated circuit protection against overvoltages. FIG. 1A is a section view according to line A--A of FIG. 1B.
One can see in FIG. 1A an integrated circuit contact pad constituted for example by a metallization 1 formed on a polycrystalline silicon layer 2 and insulated from a monocrystalline silicon substrate 3 by an oxide layer 4. This pad is in contact through an opening 5 with a highly doped region 6 according to the conductivity type opposite to that of the substrate, for example N.sup.+ if the substrate is of the P.sup.- -type. Another metallization 7 is in contact through another opening 8 with another N.sup.+ -type region 9 in register with region 6 as it clearly appears from the top view of FIG. 1B. Thus, there is between the pad metallization 1 and the metallization 7 connected to the reference voltage a double avalanche diode, the breakdown voltage of which is determined by the doping levels of the N.sup.+ and P.sup.- areas.
FIG. 1B shows, with the same references as in FIG. 1A, a top view of the structure. It is further to be noted that the N.sup.+ -type doped region 6 connected to the pad extends toward another metallization 10 which is in turn connected to the integrated circuit access with which it is desirable that the pad 1 establishes a contact and which is to be protected from overvoltages.
The equivalent circuit diagram of this structure is illustrated in FIG. 1C. The pad 1 is connected to the metallization 10 through a resistor R corresponding to the resistor of the extended portion of region 6 and this pad is connected to a reference voltage, usually the ground, through a double avalanche diode D corresponding to the N.sup.+ P.sup.- N.sup.+ structure.
This device satisfactorily operates but presents the drawback that the pad voltage is limited during an overvoltage to the voltage value of the double avalanche diode which in turn has to be higher than the normal operating values of the voltages applied to the circuit. Therefore, a damaging energy dissipation occurs in the region corresponding to the double avalanche diode, giving rise to an overheating of the circuit. Indeed, the dissipated energy is equal to the avalanche voltage value multiplied by the current flowing through the protection device. If this voltage value is reduced, the energy dissipation in the substrate will be lower, the dissipated current being roughly always the same.